The reactivity of the thirst mechanism to osmotic stimuli was examined in conscious dogs 1. under control conditions, 2. after raising the plasma vasopressin (PADH) level to about 30 muU/ml by intravenous infusion of the hormone, 3. after expansion of the blood volume by 15% by an intravenous infusion of dextran solution, and 4. after a simultaneous increase of PADH and blood-volume expansion. The osmotic thirst threshold was significantly lowered by the elevation of PADH and augmented by an expansion of blood volume, whereas no significant changes were observed when the increase in PADH and expansion of blood volume were applied simultaneously. The interactions between body-fluid osmolarity, blood volume, and vasopressin in regulation of water intake are discussed.